<!DOCTYPE html>
<html>
<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 
	<title>label</title>
	<style type="text/css">
	* {
		padding: 0;
		margin: 0;
	}

    </style>
</head>
<body>
	<div class="container">
		<h1>label</h1>
		<div id="box">
			<ul id="ul">
				<li>1</li>
				<li>2</li>
				<li>3</li>
			</ul>
		</div>
	</div>

<script type="text/javascript"> 
let lis = ul.querySelectorAll('li');
for(let li of lis) {
	li.addEventListener('click', function() {
		this.style.background = 'orange';
	})
}


</script>
</body>
</html>